End-to-end tax engagement for Illinois sole proprietors and single-member LLCs: federal Schedule C profit computation, Illinois IL-1040 at the flat 4.95% rate with Schedule M modifications, quarterly IL-1040-ES estimated payments, and entity-structure analysis comparing sole prop vs. S-corp PPRT exposure.
Establish the client's filing status, Illinois residency, and entity structure. Determine whether the business is a Schedule C sole proprietorship, a disregarded single-member LLC, or an entity that has elected S-corp or partnership treatment — because sole props and disregarded SMLLCs are exempt from Illinois PPRT (35 ILCS 5/201), while S-corps owe 1.5% PPRT. Confirm the tax year and whether a prior-year IL-1040 exists for the prior-year safe-harbour calculation.
Assemble gross receipts, cost of goods sold (if applicable), and all ordinary and necessary business deductions to arrive at net Schedule C profit. This is the federal starting point; Illinois will inherit this figure through federal AGI. Pay particular attention to home-office deduction (Form 8829), vehicle mileage or actual-expense log, and Section 179 / bonus depreciation elections — Illinois decouples from IRC §168(k) bonus depreciation and will require an add-back on Schedule M.
Complete the federal Form 1040 through the AGI line, computing self-employment tax on Schedule SE and claiming the deductible half as an above-the-line AGI deduction. Also consider the QBI deduction under IRC §199A (up to 20% of qualified business income for eligible sole proprietors). The resulting federal AGI is the mandatory starting point for the Illinois IL-1040 computation.
Start from federal AGI and apply Illinois Schedule M additions and subtractions to arrive at Illinois base income. Key additions: any IRC §168(k) bonus depreciation claimed federally must be added back (35 ILCS 5/203(a)(2)(D-25)); out-of-state municipal bond interest must be added. Key subtractions: U.S. government obligation interest, Illinois income tax refund included in federal AGI. Apply the 2025 personal exemption ($2,850 single / $5,700 MFJ per IDOR FY2025-16) and compute tax at the flat 4.95% rate. Then apply credits: Illinois Property Tax Credit (5% of property taxes paid on principal Illinois residence, Schedule ICR) and Illinois Earned Income Credit (20% of federal EIC, Schedule IL-E/EIC).
Determine whether quarterly Illinois estimated payments are required (threshold: $1,000 of expected Illinois tax after withholding and credits per 35 ILCS 5/803 for TY2025) and compute the safe-harbour amount — the lesser of 90% of current-year estimated liability or 100% of the prior-year Illinois tax. Divide by four for equal quarterly installments due April 15, June 15, September 15 (current year), and January 15 (following year). Payments can be made electronically through MyTax Illinois without mailing Form IL-1040-ES vouchers.
For clients with net profit above approximately $50,000–$60,000, compare the total Illinois tax cost of the current sole-proprietorship structure against an S-corporation election. A sole proprietor or disregarded SMLLC pays 0% Illinois PPRT and 4.95% Illinois PIT on all net income. An S-corp pays 1.5% PPRT at the entity level on Illinois-allocated net income (IL-1120-ST) plus 4.95% PIT on the pass-through; however, reasonable W-2 salary reduces PPRT base while the S-corp may also elect the 4.95% PTE tax under Public Act 102-0658 for federal SALT-cap relief. Present the Illinois-side numbers and flag for federal SE tax analysis.
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